Tachyon: Reliable, Memory Speed Storage for Cluster
Computing Frameworks
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Citation
Haoyuan Li, Ali Ghodsi, Matei Zaharia, Scott Shenker, and Ion
Stoica. 2014. Tachyon: Reliable, Memory Speed Storage for
Cluster Computing Frameworks. In Proceedings of the ACM
Symposium on Cloud Computing (SOCC '14). ACM, New York,
NY, USA, Article 6 , 15 pages.
As Published
http://dx.doi.org/10.1145/2670979.2670985
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Thu May 26 19:40:16 EDT 2016
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Tachyon: Reliable, Memory Speed Storage
for Cluster Computing Frameworks
Haoyuan Li
Ali Ghodsi
University of California, Berkeley
{haoyuan,alig}@cs.berkeley.edu
Matei Zaharia
MIT, Databricks
[email protected]
Abstract
Tachyon is a distributed file system enabling reliable data
sharing at memory speed across cluster computing frameworks. While caching today improves read workloads,
writes are either network or disk bound, as replication is
used for fault-tolerance. Tachyon eliminates this bottleneck
by pushing lineage, a well-known technique, into the storage
layer. The key challenge in making a long-running lineagebased storage system is timely data recovery in case of failures. Tachyon addresses this issue by introducing a checkpointing algorithm that guarantees bounded recovery cost
and resource allocation strategies for recomputation under
commonly used resource schedulers. Our evaluation shows
that Tachyon outperforms in-memory HDFS by 110x for
writes. It also improves the end-to-end latency of a realistic
workflow by 4x. Tachyon is open source and is deployed at
multiple companies.
1.
Introduction
Over the past few years, there have been tremendous efforts
to improve the speed and sophistication of large-scale dataparallel processing systems. Practitioners and researchers
have built a wide array of programming frameworks [34–
36, 42, 53, 54] and storage systems [14, 15, 25, 26, 39]
tailored to a variety of workloads.
As the performance of many of these systems is I/O
bound, traditional means of improving their speed is to
cache data into memory [12]. While caching can dramatically improve read performance, unfortunately, it does not
help much with write performance. This is because highly
parallel systems need to provide fault-tolerance, and the way
they achieve it is by replicating the data written across nodes.
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SoCC ’14, 3-5 Nov. 2014, Seattle, Washington, USA.
ACM 978-1-4503-3252-1.
http://dx.doi.org/10.1145/2670979.2670985
Scott Shenker
Ion Stoica
University of California, Berkeley
{shenker,istoica}@cs.berkeley.edu
Even replicating the data in memory can lead to a significant drop in the write performance, as both the latency and
throughput of the network are typically much worse than
that of local memory.
Slow writes can significantly hurt the performance of job
pipelines, where one job consumes the output of another.
These pipelines are regularly produced by workflow managers such as Oozie [4] and Luigi [7], e.g., to perform data
extraction with MapReduce, then execute a SQL query, then
run a machine learning algorithm on the query’s result. Furthermore, many high-level programming interfaces [5, 8,
46], such as Pig [38] and FlumeJava [19], compile programs
into multiple MapReduce jobs that run sequentially. In all
these cases, data is replicated across the network in-between
each of the steps.
To improve write performance, we present Tachyon, an
in-memory storage system that achieves high throughput
writes and reads, without compromising fault-tolerance.
Tachyon circumvents the throughput limitations of replication by leveraging the concept of lineage, where lost output
is recovered by re-executing the operations (tasks) that created the output. As a result, lineage provides fault-tolerance
without the need for replicating the data.
While the concept of lineage has been used before in
the context of frameworks such as Spark, Nectar and BADFS [16, 27, 53], there are several important challenges that
arise when pushing it into a continuously running distributed
storage system. Previous systems do not address these challenges.
The first challenge is bounding the recomputation cost
for a long-running storage system. This challenge does not
exist for a single computing job, such as a MapReduce
or Spark job, as in this case, the recomputation time is
trivially bounded by the job’s execution time. In contrast,
Tachyon runs indefinitely, which means that the recomputation time can be unbounded. Previous frameworks that support long running workloads, such as Spark Streaming [54]
and BAD-FS [16], circumvent this challenge by using periodic checkpointing. Unfortunately, using the same techniques in Tachyon is difficult, as the storage layer is agnostic
to the semantics of the jobs running on the data (e.g., when
outputs will be reused), and job execution characteristics can
vary widely. In particular, any fixed checkpoint interval can
lead to unbounded recovery times if data is written faster
than the available disk bandwidth. Instead, we prefer to select which data to checkpoint based on the structure of the
lineage graph in a manner that still bounds recovery time.
The second challenge is how to allocate resources for recomputations. For example, if jobs have priorities, Tachyon
must, on the one hand, make sure that recomputation tasks
get adequate resources (even if the cluster is fully utilized),
and on the other hand, Tachyon must ensure that recomputation tasks do not severely impact the performance of currently running jobs with possibly higher priorities.
Tachyon bounds the data recomputation cost, thus addressing the first challenge, by continuously checkpointing
files asynchronously in the background. To select which
files to checkpoint and when, we propose a novel algorithm,
called the Edge algorithm, that provides an upper bound on
the recomputation cost regardless of the workload’s access
pattern.
To address the second challenge, Tachyon provides resource allocation schemes that respect job priorities under
two common cluster allocation models: strict priority and
weighted fair sharing [31, 52]. For example, in a cluster using a strict priority scheduler, if a missing input is requested
by a low priority job, the recomputation minimizes its impact on high priority jobs. However, if the same input is later
requested by a higher priority job, Tachyon automatically increases the amount of resources allocated for recomputation
to avoid priority inversion [33].
We have implemented Tachyon with a general lineagespecification API that can capture computations in many
of today’s popular data-parallel computing models, e.g.,
MapReduce and SQL. We also ported the Hadoop and Spark
frameworks to run on top of it1 . The project is open source,
has more than 40 contributors from over 15 institutions, and
is deployed at multiple companies.
Our evaluation shows that on average, Tachyon2 achieves
110x higher write throughput than in-memory HDFS [2].
In a realistic industry workflow, Tachyon improves end-toend latency by 4x compared to in-memory HDFS. In addition, because many files in computing clusters are temporary
files that get deleted before they are checkpointed, Tachyon
can reduce replication-caused network traffic by up to 50%.
Finally, based on traces from Facebook and Bing, Tachyon
would consume no more than 1.6% of cluster resources for
recomputation.
More importantly, due to the inherent bandwidth limitations of replication, a lineage-based recovery model might
1 Applications
can choose to use the lineage feature or not by a configuration parameter.
2 This paper focus on in-memory Tachyon deployment. However, Tachyon
can also speed up SSD- and disk-based systems if the aggregate local I/O
bandwidth is higher than the network bandwidth.
Media
HDD (x12)
SDD (x4)
Network
Memory
Capacity
12-36 TB
1-4 TB
N/A
128-512 GB
Bandwith
0.2-2 GB/sec
1-4 GB/sec
1.25 GB/sec
10-100 GB/sec
Table 1: Typical datacenter node setting [6].
be the only way to make cluster storage systems match the
speed of in-memory computations in the future. This work
aims to address some of the leading challenges in making
such a system possible.
2.
Background
This section describes our target workloads and provides
background on existing solutions. Section 8 describes related work in greater detail.
2.1
Target Workload Properties
We have designed Tachyon targeting today’s big data workloads, which have the following properties:
• Immutable data: Data is immutable once written, since
dominant underlying storage systems, such as HDFS [2],
only support the append operation.
• Deterministic jobs: Many frameworks, such as MapReduce [23] and Spark [53], use recomputation for fault
tolerance within a job and require user code to be deterministic. We provide lineage-based recovery under the
same assumption. Nondeterministic frameworks can still
store data in Tachyon using replication.
• Locality based scheduling: Many computing frameworks [23, 53] schedule jobs based on locality to minimize network transfers, so reads can be data-local.
• All data vs. working set: Even though the whole data set
is large and has to be stored on disks, the working set of
many applications fits in memory [12, 53].
• Program size vs. data size: In big data processing, the
same operation is repeatedly applied on massive data.
Therefore, replicating programs is much less expensive
than replicating data in many cases.
2.2
The Case Against Replication
In-memory computation frameworks – such as Spark and
Piccolo [42], as well as caching in storage systems – have
greatly sped up the performance of individual jobs. However,
sharing (writing) data reliably among different jobs often
becomes a bottleneck.
The write throughput is limited by disk (or SSD) and
network bandwidths in existing big data storage solutions,
such as HDFS [2], FDS [14], Cassandra [1], HBase [3], and
RAMCloud [39]. All these systems use media with much
lower bandwidth than memory (Table 1).
The fundamental issue is that in order to be fault-tolerant,
these systems replicate data across the network and write
Standby Master
ZooKeeper
Quorum
Standby
Tachyon Master
Workflow
Manager
Standby Master
Standby
File
Set B
MapReduce Job
File
Set C
Tachyon Worker
Worker Daemon
Spark Job
File
Set D
File
Set E
Ramdisk
Tachyon Worker
Worker Daemon
Figure 2: A lineage graph example of multiple frameworks
Ramdisk
at least one copy onto non-volatile media to allow writes
to survive datacenter-wide failures, such as power outages.
Because of these limitations and the advancement of inmemory computation frameworks [34, 35, 42, 53], inter-job
data sharing cost often dominates pipeline’s end-to-end latencies for big data workloads. While some jobs’ outputs are
much smaller than their inputs, a recent trace from Cloudera
showed that, on average, 34% of jobs (weighted by compute
time) across five customers had outputs that were at least as
large as their inputs [20]. In an in-memory computing cluster, these jobs would be write throughput bound.
Hardware advancement is unlikely to solve the issue.
Memory bandwidth is one to three orders of magnitude
higher than the aggregate disk bandwidth on a node. The
bandwidth gap between memory and disk is becoming
larger. The emergence of SSDs has little impact on this
since its major advantage over disk is random access latency, but not sequential I/O bandwidth, which is what most
data-intensive workloads need. Furthermore, throughput increases in network indicate that over-the-network memory
replication might be feasible. However, sustaining datacenter power outages requires at least one disk copy for the
system to be fault-tolerant. Hence, in order to provide high
throughput, storage systems have to achieve fault-tolerance
without replication.
Design Overview
This section overviews the design of Tachyon, while the
following two sections (§4 & §5) focus on the two main
challenges that a storage system incorporating lineage faces:
bounding recovery cost and allocating resources for recomputation.
3.1
Spark Job
Ramdisk
Figure 1: Tachyon Architecture.
3.
File
Set A
Tachyon Worker
Worker Daemon
System Architecture
Tachyon consists of two layers: lineage and persistence. The
lineage layer provides high throughput I/O and tracks the sequence of jobs that have created a particular data output. The
persistence layer persists data onto storage without the lineage concept. This is mainly used to do asynchronous checkpoints. The persistence layer can be any existing replication
based storage systems, such as HDFS, S3, Glusterfs.
Tachyon employs a standard master-slave architecture
similar to HDFS and GFS (see Figure 1). In the remainder
of this section we discuss the unique aspects of Tachyon.
In addition to managing metadata, the master also contains a workflow manager. The role of this manager is to
track lineage information, compute checkpoint order (§4),
and interact with a cluster resource manager to allocate resources for recomputation (§5).
Each worker runs a daemon that manages local resources,
and periodically reports the status to the master. In addition,
each worker uses a RAMdisk for storing memory-mapped
files. A user application can bypass the daemon and interact
directly with RAMdisk. This way, an application with data
locality §2.1 can interact with data at memory speeds, while
avoiding any extra data copying.
3.2
An Example
To illustrate how Tachyon works, consider the following example. Assume job P reads file set A and writes file set B.
Before P produces the output, it submits its lineage information L to Tachyon. This information describes how to run
P (e.g., command line arguments, configuration parameters)
to generate B from A. Tachyon records L reliably using the
persistence layer. L guarantees that if B is lost, Tachyon can
recompute it by (partially3 re-executing P. As a result, leveraging the lineage, P can write a single copy of B to memory without compromising fault-tolerance. Figure 2 shows a
more complex lineage example.
Recomputation based recovery assumes that input files
are immutable (or versioned, c.f., §9) and that the executions
of the jobs are deterministic. While these assumptions are
not true of all applications, they apply to a large fraction
of datacenter workloads (c.f., §2.1), which are deterministic
applications (often in a high-level language such as SQL
where lineage is simple to capture).
Tachyon improves write performance by avoiding replication. However, replication has the advantage of improving
read performance as more jobs read the same data. While
Tachyon leverages lineage to avoid data replication, it uses a
client-side caching technique to mitigate read hotspots transparently. That is, if a file is not available on the local machine, it is read from a remote machine and cached locally
in Tachyon, temporarily increasing its replication factor.
3 The
recomputation granularity depends on the framework integration. For
example, it can be job level or task level.
Return
Global
Unique
Lineage Id
Dependency Info
Signature
createDependency(inputFiles, outputFiles, binaryPrograms, executionConfiguration, dependencyType)
getDependency(lineageId)
Table 2: Submit and Retrieve Lineage API
3.3
API Summary
Tachyon is an append-only file system, similar to HDFS, that
supports standard file operations, such as create, open, read,
write, close, and delete. In addition, Tachyon provides an
API to capture the lineage across different jobs and frameworks. Table 2 lists the lineage API, and Section 6.1 describes this API in detail.
The lineage API adds complexity to Tachyon over replication based file systems such as HDFS, S3. However, only
framework programmers need to understand Tachyon’s lineage API. Tachyon does not place extra burden on application programmers. As long as a framework, e.g. Spark,
integrates with Tachyon, applications on top of the framework take advantage of lineage based fault-tolerance transparently4 . Furthermore, a user can choose to use Tachyon as
a traditional file system if he/she does not use the lineage
API. In this case, the application would not have the benefit
of memory throughput writes, but will still perform no worse
than on a traditional replicated file system.
3.4
Lineage Overhead
In terms of storage, job binaries represent by far the largest
component of the lineage information. However, according
to Microsoft data [27], a typical data center runs 1, 000
jobs daily on average, and it takes up to 1 TB to store the
uncompressed binaries of all jobs executed over a one year
interval. This overhead is negligible even for a small sized
data center.
Furthermore, Tachyon can garbage collect the lineage information. In particular, Tachyon can delete a lineage record
after checkpointing (c.f., §4) its output files. This will dramatically reduce the overall size of the lineage information.
In addition, in production environments, the same binary
program is often executed many times, e.g., periodic jobs,
with different parameters. In this case, only one copy of the
program needs to be stored.
3.5
Data Eviction
Tachyon works best when the workload’s working set fits
in memory. In this context, one natural question is what is
the eviction policy when the memory fills up. Our answer
to this question is influenced by the following characteristics identified by previous works [20, 44] for data intensive
applications:
4 We
made this configurable in our Spark and MapReduce integration,
which means applications on top of them can choose not to use this feature.
• Access Frequency: File access often follows a Zipf-like
distribution (see [20, Figure 2]).
• Access Temporal Locality: 75% of the re-accesses take
place within 6 hours (see [20, Figure 5]).
Based on these characteristics, we use LRU as a default
policy. However, since LRU may not work well in all scenarios, Tachyon also allows plugging in other eviction policies.
Finally, as we describe in Section 4, Tachyon stores all but
the largest files in memory. The rest are stored directly to the
persistence layer.
3.6
Master Fault-Tolerance
As shown in Figure 1, Tachyon uses a “passive standby” approach to ensure master fault-tolerance. The master logs every operation synchronously to the persistence layer. When
the master fails, a new master is selected from the standby
nodes. The new master recovers the state by simply reading the log. Note that since the metadata size is orders of
magnitude smaller than the output data size, the overhead of
storing and replicating it is negligible.
3.7
Handling Environment Changes
One category of problems Tachyon must deal with is changes
in the cluster’s runtime environment. How can we rely on
re-executing binaries to recompute files if, for example, the
version of the framework that an application depends on
changes, or the OS version changes?
One observation we make here is that although files’ dependencies may go back in time forever, checkpointing allows us to place a bound on how far back we ever have to
go to recompute data. Thus, before an environment change,
we can ensure recomputability by switching the system into
a “synchronous mode”, where (a) all currently unreplicated
files are checkpointed and (b) all new data is saved synchronously. Once the current data is all replicated, the update
can proceed and this mode can be disabled 5 .
For more efficient handling of this case, it might also be
interesting to capture a computation’s environment using a
VM image [28]. We have, however, not yet explored this
option.
3.8
Why Storage Layer
To achieve memory throughput I/O, we need to avoid replication and thus use lineage information. We believe that it is
necessary to push lineage into the storage layer because of
the followings:
First, a data processing pipeline usually contains more
than one job. It is necessary to capture the lineage across
jobs. Furthermore, in a production environment, the data
producer and its consumers could be written in different
frameworks. Therefore, only understanding the lineage at
5 Data
deletion can be handled as a similar approach. For example, when
a user tries to delete a file A, if there are other files depending on A,
Tachyon will delete A asynchronously after the depending data has been
checkpointed.
the job level or a single framework level can not solve the
issue.
Second, only the storage layer knows when files are renamed or deleted, which is necessary to track lineage correctly and checkpoint data correctly in long-term operations.
Since other layers do not have full control on the storage
layer, it is possible that a user may manually delete a file.
All files depending on the deleted file will lose fault tolerance guarantee.
4.
Checkpointing
This section outlines the checkpoint algorithm used by
Tachyon to bound the amount of time it takes to retrieve
a file that is lost due to failures6 . By a file we refer to a
distributed file, e.g., all output of a MapReduce/Spark job.
Unlike other frameworks, such as MapReduce and Spark,
whose jobs are relatively short-lived, Tachyon runs continuously. Thus, the lineage that accumulates can be substantial,
requiring long recomputation time in the absence of checkpoints. Therefore, checkpointing is crucial for the performance of Tachyon. Note that long-lived streaming systems,
such as Spark Streaming [54], leverage their knowledge
of job semantics to decide what and when to checkpoint.
Tachyon has to checkpoint in absence of such detailed semantic knowledge.
The key insight behind our checkpointing approach in
Tachyon is that lineage enables us to asynchronously checkpoint in the background, without stalling writes, which can
proceed at memory-speed. This is unlike other storage systems that do not have lineage information, e.g., key-value
stores, which synchronously checkpoint, returning to the application that invoked the write only once data has been persisted to stable storage. Tachyon’ background checkpointing
is done in a low priority process to avoid interference with
existing jobs.
An ideal checkpointing algorithm would provide the following:
1. Bounded Recomputation Time. Lineage chains can grow
very long in a long-running system like Tachyon, therefore the checkpointing algorithm should provide a bound
on how long it takes to recompute data in the case of
failures. Note that bounding the recomputation time also
bounds the computational resources used for recomputations.
2. Checkpointing Hot files. Some files are much more popular than others. For example, the same file, which represents a small dimension table in a data-warehouse, is
repeatedly read by all mappers to do a map-side join with
a fact table [12].
3. Avoid Checkpointing Temporary Files. Big data workloads generate a lot of temporary data. From our contacts
at Facebook, nowadays, more than 70% data is deleted
within a day, without even counting shuffle data. Figure 3a illustrates how long temporary data exists in a
cluster at Facebook7 . An ideal algorithm would avoid
checkpointing much of this data.
We consider the following straw man to motivate our
algorithm: asynchronously checkpoint every file in the order
that it is created. Consider a lineage chain, where file A1 is
used to generate A2 , which is used to generate A3 , A4 , and
so on. By the time A6 is being generated, perhaps only A1
and A2 have been checkpointed to stable storage. If a failure
occurs, then A3 through A6 have to be recomputed. The
longer the chain, the longer the recomputation time. Thus,
spreading out checkpoints throughout the chain would make
recomputations faster.
4.1
Edge Algorithm
Based on the above characteristics, we have designed a simple algorithm, called Edge, which builds on three ideas.
First, Edge checkpoints the edge (leaves) of the lineage
graph (hence the name). Second, it incorporates priorities,
favoring checkpointing high-priority files over low-priority
ones. Finally, the algorithm only caches datasets that can
fit in memory to avoid synchronous checkpointing, which
would slow down writes to disk speed. We discuss each of
these ideas in detail:
Checkpointing Leaves. The Edge algorithm models the
relationship of files with a DAG, where the vertices are files,
and there is an edge from a file A to a file B if B was
generated by a job that read A. The algorithm checkpoints
the latest data by checkpointing the leaves of the DAG. This
lets us satisfy the requirement of bounded recovery time
(explained in Section 4.2).
Figure 4 illustrates how the Edge algorithm works. At the
beginning, there are only two jobs running in the cluster,
generating files A1 and B1 . The algorithm checkpoints both
of them. After they have been checkpointed, files A3 , B4 , B5 ,
and B6 become leaves. After checkpointing these, files A6 ,
B9 become leaves.
To see the advantage of Edge checkpointing, consider the
pipeline only containing A1 to A6 in the above example. If a
failure occurs when A6 is being checkpointed, Tachyon only
needs to recompute from A4 through A6 to get the final result.
As previously mentioned, checkpointing the earliest files,
instead of the edge, would require a longer recomputation
chain.
This type of pipeline is common in industry. For example, continuously monitoring applications generate hourly
reports based on minutely reports, daily reports based on
hourly reports, and so on.
6 In
this section, we assume recomputation has the same resource as the
first time computation. In Section 5, we address the recomputation resource
allocation issue.
7 The
workload was collected from a 3,000 machine MapReduce cluster at
Facebook, during a week in October 2010.
Access Count
Percentage
1
62%
3
86%
5
93%
A1
10
95%
Output Data Size (GB)
0.8
CDF
0.6
0.4
0.2
0.0
100
104
101
102
103
105
Data Existence Duration (sec)
18000
16000
14000
12000
10000
8000
6000
4000
2000
0Fri
Sat
Sun
Mon
Time
A1
A3
Tue
Wed
(a) Estimated temporary data (b) Data generation rates at five
span including shuffle data
minutes granularity
Figure 3: A 3,000 node MapReduce cluster at Facebook
Checkpointing Hot Files. The above idea of checkpointing the latest data is augmented to first checkpoint high priority files. Tachyon assigns priorities based on the number
of times a file has been read. Similar to the LFU policy for
eviction in caches, this ensures that frequently accessed files
are checkpointed first. This covers the case when the DAG
has a vertex that is repeatedly read leading to new vertices
being created, i.e., a high degree vertex. These vertices will
be assigned a proportionally high priority and will thus be
checkpointed, making recovery fast.
Edge checkpointing has to balance between checkpointing leaves, which guarantee recomputation bounds, and
checkpointing hot files, which are important for certain iterative workloads. Here, we leverage the fact that most big
data workloads have a Zipf-distributed popularity (this has
been observed by many others [12, 20]). Table 3 shows what
percentage of the files are accessed less than (or equal) than
some number of times in a 3,000-node MapReduce cluster
at Yahoo in January 2014. Based on this, we consider a file
high-priority if it has an access count higher than 2. For this
workload, 86% of the checkpointed files are leaves, whereas
the rest are non-leaf files. Hence, in most cases bounds can
be provided. The number can naturally be reconfigured for
other workloads. Thus, files that are accessed more than
twice get precedence in checkpointing compared to leaves.
A replication-based filesystem has to replicate every file,
even temporary data used between jobs. This is because failures could render such data as unavailable. Tachyon avoids
checkpointing much of the temporary files created by frameworks. This is because checkpointing later data first (leaves)
or hot files, allows frameworks or users to delete temporary
data before it gets checkpointed8 .
Dealing with Large Data Sets. As observed previously,
working sets are Zipf-distributed [20, Figure 2]. We can
A1
B1
A2
Table 3: File Access Frequency at Yahoo
1.0
B1
B2
B4
B3
B5
B6
A2
A3
B2
B4
B3
B5
B6
A4
B7
A5
B8
A6
B9
Figure 4: Edge Checkpoint Example. Each node represents
a file. Solid nodes denote checkpointed files, while dotted
nodes denote uncheckpointed files.
therefore store in memory all but the very largest datasets,
which we avoid storing in memory altogether. For example,
the distribution of input sizes of MapReduce jobs at Facebook is heavy-tailed [11, Figure 3a]. Furthermore, 96% of
active jobs can have their entire data simultaneously fit in
the corresponding clusters’ memory [11]. The Tachyon master is thus configured to synchronously write datasets above
the defined threshold to disk. In addition, Figure 3b shows
that file requests in the aforementioned Facebook cluster
is highly bursty. During bursts, Edge checkpointing might
checkpoint leafs that are far apart in the DAG. As soon as
the bursts finish, Edge checkpointing starts checkpointing
the rest of the non-leaf files. Thus, most of the time most
of the files in memory have been checkpointed and can be
evicted from memory if room is needed (see Section 3). If
the memory fills with files that have not been checkpointed,
Tachyon checkpoints them synchronously to avoid having
to recompute long lineage chains. In summary, all but the
largest working sets are stored in memory and most data has
time to be checkpointed due to the bursty behavior of frameworks. Thus, evictions of uncheckpointed files are rare.
4.2
Bounded Recovery Time
Checkpointing the edge of the DAG lets us derive a bound
on the recomputation time. The key takeaway of the bound
is that recovery of any file takes on the order of time that it
takes to read or generate an edge. Informally, it is independent of the depth of the lineage DAG.
Recall that the algorithm repeatedly checkpoints the edge
of the graph. We refer to the time it takes to checkpoint a
particular edge i of the DAG as Wi . Similarly, we refer to the
time it takes to generate an edge i from its ancestors as Gi .
We now have the following bound.
T HEOREM 1. Edge checkpointing ensures that any file can
be recovered in 3×M, for M = maxi {Ti }, Ti = max(Wi , Gi )9 .
This shows that recomputations are independent of the
“depth” of the DAG. This assumes that the caching behavior
is the same during the recomputation, which is true when
working sets fit in memory (c.f., Section 4.1).
8 In
our implementation, Tachyon also allows frameworks to indicate temporary data explicitly by path name.
B1
9 We
refer the reader to [32] for the proof.
The above bound does not apply to priority checkpointing. However, we can easily incorporate priorities by alternating between checkpointing the edge c fraction of the time
and checkpointing high-priority data 1 − c of the time.
C OROLLARY 2. Edge checkpointing, where c fraction of the
time is spent checkpointing the edge, ensures that any file can
be recovered in 3×M
c , for M = maxi {Ti }, Ti = max(Wi , Gi ).
Thus, configuring c = 0.5 checkpoints the edge half of
the time, doubling the bound of Theorem 1. These bounds
can be used to provide SLOs to applications.
In practice, priorities can improve the recomputation cost.
In the evaluation(§7), we illustrate actual recomputation
times when using edge caching.
5.
Resource Allocation
Although the Edge algorithm provides a bound on recomputation cost, Tachyon needs a resource allocation strategy
to schedule jobs to recompute data in a timely manner. In
addition, Tachyon must respect existing resource allocation
policies in the cluster, such as fair sharing or priority.
In many cases, there will be free resources for recomputation, because most datacenters are only 30–50% utilized.
However, care must be taken when a cluster is full. Consider a cluster fully occupied by three jobs, J1 , J2 , and J3 ,
with increasing importance (e.g., from research, testing, and
production). There are two lost files, F1 and F2 , requiring recomputation jobs R1 and R2 . J2 requests F2 only. How should
Tachyon schedule recomputations?
One possible solution is to statically assign part of the
cluster to Tachyon, e.g., allocate 25% of the resources on the
cluster for recomputation. However, this approach limits the
cluster’s utilization when there are no recomputation jobs. In
addition, the problem is complicated because many factors
can impact the design. For example, in the above case, how
should Tachyon adjust R2 ’s priority if F2 is later requested
by the higher priority job J3 ?
To guide our design, we identify three goals:
1. Priority compatibility: If jobs have priorities, recomputation jobs should follow them. For example, if a file is
requested by a low priority job, the recomputation should
have minimal impact on higher priority jobs. But if the
file is later requested by a high priority job, the recovery
job’s importance should increase.
2. Resource sharing: If there are no recomputation jobs, the
whole cluster should be used for normal work.
3. Avoid cascading recomputation: When a failure occurs,
more than one file may be lost at the same time. Recomputing them without considering data dependencies may
cause recursive job launching.
We start by presenting resource allocation strategies that
meet the first two goals for common cluster scheduling policies. Then, we discuss how to achieve the last goal, which is
orthogonal to the scheduling policy.
J1(P1)
J2(P2)
J3(P3)
J1(P1)
J2(P2)
R2(P2)
R2(P3)
(a)
(b)
J3(P3)
Figure 5: Resource Allocation Strategy for Priority Based
Scheduler.
5.1
Resource Allocation Strategy
The resource allocation strategy depends on the scheduling
policy of the cluster Tachyon runs on. We present solutions
for priority and weighted fair sharing, the most common
policies in systems like Hadoop and Dryad [31, 52].
Priority Based Scheduler In a priority scheduler, using
the same example above, jobs J1 , J2 , and J3 have priorities
P1 , P2 , and P3 respectively, where P1 < P2 < P3 .
Our solution gives all recomputation jobs the lowest priority by default, so they have minimal impact on other jobs.
However, this may cause priority inversion. For example, because file F2 ’s recomputation job R2 has a lower priority than
J2 , it is possible that J2 is occupying the whole cluster when
it requests F2 . In this case, R2 cannot get resources, and J2
blocks on it.
We solve this by priority inheritance. When J2 requests
F2 , Tachyon increases R2 ’s priority to be P2 . If F2 is later
read by J3 , Tachyon further increases its priority. Figure 5a
and 5b show jobs’ priorities before and after J3 requests F2 .
Fair Sharing Based Scheduler In a hierarchical fair sharing scheduler, jobs J1 , J2 , and J3 have shares W1 , W2 , and W3
respectively. The minimal share unit is 1.
In our solution, Tachyon has a default weight, WR (as
the minimal share unit 1), shared by all recomputation jobs.
When a failure occurs, all lost files are recomputed by jobs
with a equal share under WR . In our example, both R1 and R2
are launched immediately with share 1 in WR .
When a job requires lost data, part of the requesting job’s
share10 , is moved to the recomputation job. In our example,
when J2 requests F2 , J2 has share (1 − a) under W2 , and R2
share a under W2 . When J3 requests F2 later, J3 has share
1 − a under W3 and R2 has share a under W3 . When R2
finishes, J2 and J3 resumes all of their previous shares, W2
and W3 respectively. Figure 6 illustrates.
This solution fulfills our goals, in particular, priority compatibility and resource sharing. When no jobs are requesting
a lost file, the maximum share for all recomputation jobs is
bounded. In our example, it is WR /(W1 + W2 + W3 + WR ).
When a job requests a missing file, the share of the corresponding recomputation job is increased. Since the increased
share comes from the requesting job, there is no performance
impact on other normal jobs.
10 a
could be a fixed portion of the job’s share, e.g., 20%
J1
W1 W2 W3 WR(1)
J2
J3
1
1
R2
R3
W1 W2
J1
W3 WR(1)
J3
1-a a
J2
R2
1
1
R3
W1 W2 W3 WR(1)
J1
1-a
J3
a a
1-a
J2
R2
1
J1
W1 W2 W3 WR(1)
J3
J2
1
1
R3
R3
Figure 6: Resource Allocation Strategy for Fair Sharing Based Scheduler.
5.2
Recomputation Order
Recomputing a file might require recomputing other files
first, such as when a node fails and loses multiple files at the
same time. While the programs could recursively make callbacks to the workflow manager to recompute missing files,
this would have poor performance. For instance, if the jobs
are non-preemptable, computation slots are occupied, waiting for other recursively invoked files to be reconstructed.
If the jobs are preemptable, computation before loading lost
data is wasted. For these reasons, the workflow manager determines in advance the order of the files that need to be
recomputed and schedules them.
To determine the files that need to be recomputed, the
workflow manager uses a logical directed acyclic graph
(DAG) for each file that needs to be reconstructed. Each
node in the DAG represents a file. The parents of a child
node in the DAG denote the files that the child depends on.
That is, for a wide dependency a node has an edge to all files
it was derived from, whereas for a narrow dependency it has
a single edge to the file that it was derived from. This DAG
is a subgraph of the DAG in Section 4.1.
To build the graph, the workflow manager does a depthfirst search (DFS) of nodes representing targeted files. The
DFS stops whenever it encounters a node that is already
available in storage. The nodes visited by the DFS must be
recomputed. The nodes that have no lost parents in the DAG
can be recomputed first in parallel. The rest of nodes can be
recomputed when all of their children become available. The
workflow manager calls the resource manager and executes
these tasks to ensure the recomputation of all missing data.
6.
Implementation
We have implemented Tachyon in about 36,000 lines of
Java. Tachyon uses an existing storage system as its persistence layer, with a pluggable interface (we currently support
HDFS, S3, GlusterFS, and NFS). Tachyon also uses Apache
ZooKeeper to do leader election for master fault tolerance.
We have also implemented patches for existing frameworks to work with Tachyon: 300 Lines-of-Code (LoC) for
Spark [53] and 200 LoC for MapReduce [2]. Applications
on top of integrated frameworks can take advantage of the
linage transparently, and application programmers do not
need to know the lineage concept.
The project is open source, has over 40 contributors from
more than 15 companies and universities.
6.1
Lineage Metadata
This section describes the detailed information needed to
construct a lineage.
Ordered input files list: Because files’ names could be
changed, each file is identified by a unique immutable file
ID in the ordered list to ensure that the application’s potential
future recomputations read the same files in the same order
as its first time execution.
Ordered output files list: This list shares the same insights
as the input files list.
Binary program for recomputation: Tachyon launches
this program to regenerate files when necessary. There are
various approaches to implement a file recomputation program. One naı̈ve way is to write a specific program for each
application. However, this significantly burdens application
programmers. Another solution is to write a single wrapper
program which understands both Tachyon’s lineage information and the application’s logic. Though this may not be
feasible for all programs, it works for applications written
in a particular framework. Each framework can implement
a wrapper to allow applications written in the framework
to use Tachyon transparently. Therefore, no burden will be
placed on application programmers.
Program configuration: Program configurations can be
dramatically different in various jobs and frameworks. We
address this by having Tachyon forego any attempt to understand these configurations. Tachyon simply views them
as byte arrays, and leaves the work to program wrappers to
understand. Based on our experience, it is fairly straightforward for each framework’s wrapper program to understand
its own configuration. For example, in Hadoop, configurations are kept in HadoopConf, while Spark stores these in
SparkEnv. Therefore, their wrapper programs can serialize
them into byte arrays during lineage submission, and deserialize them during recomputation.
Dependency type: We use wide and narrow dependencies
for efficient recovery(c.f., §5). Narrow dependencies represent programs that do operations, e.g., filter and map, where
each output file only requires one input file. Wide dependencies represent programs that do operations, e.g., shuffle and
Framework Integration
When a program written in a framework runs, before it
writes files, it provides the aforementioned information
(§6.1) to Tachyon. Then, when the program writes files,
Tachyon recognizes the files contained in the lineage. Therefore, the program can write files to memory only, and
Tachyon relies on the lineage to achieve fault tolerance.
If any file gets lost, and needs to be recomputed, Tachyon
launches the binary program, a wrapper under a framework
invoking user application’s logic, which is stored in the corresponding lineage instance, and provides the lineage information as well as the list of lost files to the recomputation
program to regenerate the data.
Recomputation Granularity Framework integration can
be done at different granularity, with different integration
complexity and recovery efficiency. An easy but less efficient integration is to recompute data at the job level. Suppose a Map job produces ten files and one file is lost. For
job level integration, Tachyon launches the corresponding
wrapper job to recompute all ten files. Another solution is
to recompute data at the task (within a job) level, which is
harder but more efficient than the above approach. With the
same example, for task level integration, Tachyon launches
the corresponding wrapper job to only recompute the lost
file. Our integrations with MapReduce and Spark are at task
level.
Configurable Feature Though applications on an integrated framework can have memory throughput writes transparently, we make this feature optional, which is configurable by applications. This is useful if the application does
not tolerate any temporary unavailability for the generated
data.
7.
Evaluation
We evaluated Tachyon11 through a series of raw benchmarks
and experiments based on real-world workloads.
Unless otherwise noted, our experiments ran on an Amazon EC2 cluster with 10 Gbps Ethernet. Each node had 32
cores, 244GB RAM, and 240GB of SSD. We used Hadoop
(2.3.0) and Spark (0.9).
We compare Tachyon with an in-memory installation of
Hadoop’s HDFS (over RAMFS), which we dub MemHDFS.
MemHDFS still replicates data across the network for writes
but eliminates the slowdown from disk.
In summary, our results show the following:
• Performance: Tachyon can write data 110x faster than
MemHDFS. It speeds up a realistic multi-job workflow
by 4x over MemHDFS. In case of failure, it recovers in
around one minute and still finishes 3.8x faster. Tachyon
11 We
use HDFS (2.3.0) as Tachyon’s persistence layer in our evaluation.
400 MemHDFS Write 300 TheoreFcal ReplicaFon (2 copies) Based Write 200 Read Throughput 1400 Tachyon Write Throughput (GB/Sec) 6.2
Write Throughput 500 Throughput (GB/Sec) join, where each output file requires more than one input file.
This works similarly to Spark [53].
100 Tachyon Read MemHDFS Read MemHDFS Short Circuit Read HDFS Cache Short Circuit Read 1200 1000 800 600 400 200 0 0 0 10 20 Number of Machines 30 0 10 20 Number of Machines 30 Figure 7: Tachyon and MemHDFS throughput comparison. On average, Tachyon outperforms MemHDFS 110x for
write throughput, and 2x for read throughput.
helps existing in-memory frameworks like Spark improve latency by moving storage off-heap. It recovers
from master failure within 1 second.
• Asynchronous Checkpointing: The Edge algorithm outperforms any fixed checkpointing interval. Analysis
shows that Tachyon can reduce replication caused network traffic up to 50%.
• Recomputation Impact: Recomputation has minor impact on other jobs. In addition, recomputation would
consume less than 1.6% of cluster resources in traces
from Facebook and Bing.
7.1
7.1.1
Performance
Raw Performance
We first compare Tachyon’s write and read throughputs with
MemHDFS. In each experiment, we ran 32 processes on
each cluster node to write/read 1GB each, equivalent to
32GB per node. Both Tachyon and MemHDFS scaled linearly with number of nodes. Figure 7 shows our results.
For writes, Tachyon achieves 15GB/sec/node. Despite
using 10Gbps Ethernet, MemHDFS write throughput is
0.14GB/sec/node, with a network bottleneck due to 3-way
replication for fault tolerance. We also show the theoretical
maximum performance for replication on this hardware: using only two copies of each block, the limit is 0.5GB/sec/node. On average, Tachyon outperforms MemHDFS by 110x,
and the theoretical replication-based write limit by 30x.
For reads, Tachyon achieves 38GB/sec/node. We optimized HDFS read performance using two of its most recent
features, HDFS caching and short-circuit reads. With these
features, MemHDFS achieves 17 GB/sec/node. The reason
Tachyon performs 2x better is that the HDFS API still requires an extra memory copy due to Java I/O streams.
Note that Tachyon’s read throughput was higher than
write. This happens simply because memory hardware is
generally optimized to leave more bandwidth for reads.
7.1.2
Realistic Workflow
In this experiment, we test how Tachyon performs with a realistic workload. The workflow is modeled after jobs run at
60 80 Figure 8: Performance comparison for realistic workflow.
Each number is the average of three runs. The workflow ran
4x faster on Tachyon than on MemHDFS. In case of node
failure, applications recovers in Tachyon around one minute
and still finishes 3.8x faster.
a video analytics company during one hour. It contains periodic extract, transform and load (ETL) and metric reporting
jobs. Many companies run similar workflows.
The experiments ran on a 30-node EC2 cluster. The whole
workflow contains 240 jobs in 20 batches (8 Spark jobs and
4 MapReduce jobs per batch). Each batch of jobs read 1
TB and produced 500 GB. We used the Spark Grep job to
emulate ETL applications, and MapReduce Word Count to
emulate metric analysis applications. For each batch of jobs,
we ran two Grep applications to pre-process the data. Then
we ran Word Count to read the cleaned data and compute the
final results. After getting the final results, the cleaned data
was deleted.
We measured the end-to-end latency of the workflow
running on Tachyon or MemHDFS. To simulate the real
scenario, we started the workload as soon as raw data had
been written to the system, in both Tachyon and MemHDFS
tests. For the Tachyon setting, we also measured how long
the workflow took with a node failure.
Figure 8 shows the workflow’s performance on Tachyon
and MemHDFS. The pipeline ran in 16.6 minutes on Tachyon
and 67 minutes on HDFS. The speedup is around 4x. When
a failure happens in Tachyon, the workflow took 1 more
minute, still finishing 3.8x faster than MemHDFS.
With Tachyon, the main overhead was serialization and
de-serialization since we used the Hadoop TextInputFormat.
With a more efficient serialization format, the performance
gap is larger.
7.1.3
Overhead in Single Job
When running a single job instead of a pipeline, we found
that Tachyon imposes minimal overhead, and can improve
performance over current in-memory frameworks by reducing garbage collection overheads. We use Spark as an example, running a Word Count job on one worker node.
Spark can natively cache data either as deserialized Java
objects or as serialized byte arrays, which are more compact but create more processing overhead. We compare these
modes with caching in Tachyon. For small data sizes, execution times are similar. When the data grows, however,
Tachyon storage is faster than Spark’s native modes because
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MemHDFS Tachyon (w/ failure) Tachyon (wo/ failure) Figure 9: Edge and fixed interval checkpoint recovery performance comparison.
it avoids Java memory management.12 These results show
that Tachyon can be a drop-in alternative for current inmemory frameworks. Apache Spark official release 1.0.0 already uses Tachyon as its default off-heap storage solution.
7.1.4
Master Fault Tolerance
Tachyon utilizes hot failovers to achieve fast master recovery. We tested recovery for an instance with 1 to 5 million
files, and found that the failover node resumed the master’s
role after acquiring leadership within 0.5 seconds, with a
standard deviation of 0.1 second. This performance is possible because the failover constantly updates its file metadata
based on the log of the current master.
7.2
7.2.1
Asynchronous Checkpointing
Edge Checkpointing Algorithm
We evaluate the Edge algorithm by comparing it with fixedinterval checkpointing. We simulate an iterative workflow
with 100 jobs, whose execution time follows a Gaussian
distribution with a mean of 10 seconds per job. The output
of each job in the workflow requires a fixed time of 15
seconds to checkpoint. During the workflow, one node fails
at a random time.
Figure 9 compares the average recovery time of this
workflow under Edge checkpointing with various fixed
checkpoint intervals. We see that Edge always outperforms
any fixed checkpoint interval. When too small an interval
picked, checkpointing cannot keep up with program progress
and starts lagging behind.13 If the interval is too large, then
the recovery time will suffer as the last checkpoint is too
far back in time. Furthermore, even if an optimal average
checkpoint interval is picked, it can perform worse than the
Edge algorithm, which inherently varies its interval to always match the progress of the computation and can take
into account the fact that different jobs in our workflow take
different amounts of time.
12
Although Tachyon is written in Java, it stores data in a Linux RAMFS.
is, the system is still busy checkpointing data from far in the past
when a failure happens later in the lineage graph.
13 That
We also simulated other variations of this workload, e.g.,
more than one lineage chain or different average job execution times at different phases in one chain. These simulations have a similar result, with the gap between Edge algorithm and the best fixed interval being larger in more variable
workloads.
7.2.2
Network Traffic Reduction
Data replication from the filesystem consumes almost half
the cross-rack traffic in data-intensive clusters [22]. Because
Tachyon checkpoints data asynchronously some time after
it was written, it can avoid replicating short-lived files altogether if they are deleted before Tachyon checkpoints them,
and thus reduce this traffic. Networked bounded applications
running in the same cluster can therefore leverage saved
bandwidth to have better performance.
We analyze Tachyon’s bandwidth savings via simulations
with the following parameters:
• Let T be the ratio between the time it takes to checkpoint
a job’s output and the time to execute it. This depends on
how IO-bound the application is. For example, we measured a Spark Grep program using Hadoop Text Input
format, which resulted in T = 4.5, i.e., the job runs 4.5x
faster than network bandwidth. With a more efficient binary format, T will be larger.
• Let X be the percent of jobs that output permanent data.
For example, 60% (X = 60) of generated data got deleted
within 16 minutes at Facebook (Fig. 3a).
• Let Y be the percentage of jobs that read output of
previous jobs. If Y is 100, the lineage is a chain. If Y is
0, the depth of the lineage is 1. Y is 84% in an analytics
cluster at Twitter.
Based on this information, we set X as 60 and Y as 84. We
simulated 1000 jobs using Edge checkpointing. Depending
on T, the percent of network traffic saved over replication
ranges from 40% at T = 4 to 50% at T ≥ 10.
7.3
7.3.1
Recomputation Impact
Recomputation Resource Consumption
Since Tachyon relies on lineage information to recompute
missing data, it is critical to know how many resources will
be spent on recomputation, given that failures happen every
day in large clusters. In this section, we calculate the amount
of resources spent recovering using both a mathematical
model and traces from Facebook and Bing.
We make our analysis using the following assumptions:
• Mean time to failure (MTTF) for each machine is 3
years. If a cluster contains 1000 nodes, on average, there
is one node failure per day.
• Sustainable checkpoint throughput is 200MB/s/node.
• Resource consumption is measured in machine-hours.
• In this analysis, we assume Tachyon only uses the
coarse-gained recomputation at the job level to compute
worst case, even though it supports fine-grained recomputation at task level.
Worst-case analysis In the worst case, when a node fails,
its memory contains only un-checkpointed data. This requires tasks that generate output faster than 200MB/sec:
otherwise, data can be checkpointed in time. If a machine
has 128GB memory, it requires 655 seconds (128GB /
200MB/sec) to recompute the lost data. Even if this data is
recovered serially, and of all the other machines are blocked
waiting on the data during this process (e.g., they were running a highly parallel job that depended on it), recomputation takes 0.7% (655 seconds / 24 hours) of the cluster’s running time on a 1000-node cluster (with one failure per day).
This cost scales linearly with the cluster size and memory
size. For a cluster with 5000 nodes, each with 1TB memory,
the upper bound on recomputation cost is 30% of the cluster resources, which is still small compared to the typical
speedup from Tachyon.
Real traces In real workloads, the recomputation cost is
much lower than in the worst-case setting above, because
individual jobs rarely consume the entire cluster, so a node
failure may not block all other nodes. (Another reason is that
data blocks on a failed machine can often be recomputed in
parallel, but we do not quantify this here.) Figure 10 estimates these costs based on job size traces from Facebook
and Bing (from Table 2 in [12]), performing a similar computation as above with the active job sizes in these clusters.
With the same 5000-node cluster, recomputation consumes
only up to 0.9% and 1.6% of resources at Facebook and Bing
respectively. Given most clusters are only 30–50% utilized,
this overhead is negligible.
7.3.2
Impact of Recomputation on Other Jobs
In this experiment, we show that recomputating lost data
does not noticeably impact other users’ jobs that do not depend on the lost data. The experiment has two users, each
running a Spark ETL pipeline. We ran the test three times,
and report the average. Without a node failure, both users’
pipelines executed in 85 seconds on average (standard deviation: 3s). With a failure, the unimpacted users’s execution
time was 86s (std.dev. 3.5s) and the impacted user’s time was
114s (std.dev. 5.5s).
8.
Related Work
Distributed Storage Systems In big data analytics, distributed file systems [15, 45, 49], e.g., GFS [26] and FDS [14],
and key/value stores [1, 13, 25], e.g., RAMCloud [39]
and HBase [3], replicate data to different nodes for faulttolerance. Their write throughput is bottlenecked by network bandwidth. FDS uses a fast network to achieve higher
throughput. Despite the higher cost of building FDS, its
throughput is still far from memory throughput. Tachyon
leverages the lineage concept in the storage layer to eschew replication and instead stores a single in-memory copy
Facebook Workload Analysis Percentage of the cluster computa=on resource Percentage of the cluster computa=on resource Trace Summary
Bing Workload Analysis 0.9 1000 Nodes 0.8 2000 Nodes 0.7 3000 Nodes 0.6 4000 Nodes 0.5 5000 Nodes 0.4 0.3 0.2 0.1 0 128 256 384 512 640 768 896 1024 Memory Size Per Machine (GB) 1.6 1000 Nodes 1.4 2000 Nodes 1.2 3000 Nodes 1 4000 Nodes 0.8 5000 Nodes 0.6 0.4 0.2 0 128 256 384 512 640 768 896 1024 Memory Size Per Machine (GB) Figure 10: Using the trace from Facebook and Bing, recomputation consumes up to 0.9% and 1.6% of the resource in the worst
case respectively.
of files. The Apache HDFS community is shifting towards
pushing the lineage into the system, which they claimed was
partially inspired by Tachyon [29, 43]. The proposed system
maintains materialized views in un-replicated memory and
can recompute them using the required SQL computations.
BAD-FS [16] separates out a scheduler from storage,
which provides external control to the scheduler. Users submit jobs to the scheduler through a declarative workflow language, and thus the system knows the lineage among jobs.
However, to make this practical for parallel big data engines
(e.g. Spark, MapReduce, Tez), two fundamental issues stood
out: (1) ensuring that the interaction between the scheduler
and the storage system is correct, e.g. avoiding deadlocks/priority inversion, (2) bounding recomputation time, which
otherwise in a 24/7 system can grow unbounded. For the first
problem, we provide mechanisms that avoid deadlocks/priority inversion, while respecting the policy of schedulers.
For the second problem, we provide an asynchronous checkpointing algorithm that exploits the structure of the lineage
graph to continuously perform checkpointing in the background to guarantee bounded recovery time.
Cluster Computation Frameworks Spark [53] uses lineage information within a single job or shell, all running inside a single JVM. Different queries in Spark cannot share
datasets (RDD) in a reliable and high-throughput fashion,
because Spark is a computation engine, rather than a storage
system. Our integration with Spark substantially improves
existing industry workflows of Spark jobs, as they can share
datasets reliably through Tachyon. Moreover, Spark can benefit from the asynchronous checkpointing in Tachyon, which
enables high-throughput write.
Other frameworks, such as MapReduce [23] and Dryad [30],
also trace task lineage within a job. However, as execution
engines, they do not trace relations among files, and therefore can not provide high throughput data sharing among different jobs. Like Spark, they can also integrate with Tachyon
to improve the efficiency of data sharing among different
jobs or frameworks.
Caching Systems Like Tachyon, Nectar [27] also uses the
concept of lineage, but it does so only for a specific programming framework (DryadLINQ [51]), and in the con-
text of a traditional, replicated file system. Nectar is a data
reuse system for DryadLINQ queries whose goals are to
save space and to avoid redundant computations. The former
goal is achieved by deleting largely unused files and rerunning the jobs that created them when needed. However, no
time bound is provided to retrieve deleted data. The latter
goal is achieved by identifying pieces of code that are common in different programs and reusing previously computed
files. Nectar achieves this by heavily resting on the SQL like
DryadLINQ query semantics—in particular, it needs to analyze LINQ code to determine when results may be reused—
and stores data in a replicated on-disk file system rather than
attempting to speed up data access. In contrast, Tachyon’s
goal is to provide data sharing across different frameworks
with memory speed and bounded recovery time.
PACMan [12] is a memory caching system for dataintensive parallel jobs. It explores different policies to make
data warehouse caching efficient. However, PACMan does
not improve the performance of writes, nor the first read
of any data item. Therefore, the throughput of many applications remains disk-limited—for example, a multi-job
MapReduce workflow will still need to replicate data at disk
speed between each step.
Lineage Besides the applications in the above fields, lineage
has been applied in other areas, such as scientific computing [17, 47], databases [21], and distributed computing [9,
10, 37], for applications such as providing the history of
data, which can be used for accountability, security, and
data audit. [18] and [21] provide detailed surveys of this
work. Tachyon applies lineage in a different environment to
achieve memory throughput read and write, which entails a
different set of challenges.
Checkpoint Research Checkpointing has been a rich research area. Much of the research was on using checkpoints
to minimize the re-execution cost when failures happen during long jobs. For instance, much focus was on optimal
checkpoint intervals [48, 50], as well as reducing the percheckpoint overhead [24, 40, 41]. Unlike previous work,
which uses synchronous checkpoints, Tachyon does checkpointing asynchronously in the background, which is en-
abled by using lineage information to recompute any missing data if a checkpoint fails to finish.
9.
Limitations and Future Work
Tachyon aims to improve the performance for its targeted
workloads(§2.1), and the evaluations show promising results. Although many big data clusters are running our targeted workloads, we realize that there are cases in which
Tachyon provides limited improvement, e.g., CPU or network intensive jobs. In addition, there are also challenges
that future work needs to address:
Random Access Abstractions: Data-intensive applications
run jobs that output files to storage systems like Tachyon.
Often, these files are reprocessed (e.g., to a DBMS) to allow
user-facing applications to use this data. For instance, a Web
application might recommend new friends to a user. Thus,
enabling Tachyon to provide higher-level read-only randomaccess abstractions, such as a key/value interface on top
of the existing files would shorten the pipeline and enable
output of data-intensive jobs to be immediately usable.
Mutable data: This is challenging as lineage cannot generally be efficiently stored for fine-grained random-access
updates. However, there are several directions, such as exploiting deterministic updates and batch updates.
Multi-tenancy: Memory fair sharing is an important research direction for Tachyon. Policies such as LRU/LFU
might provide good overall performance at the expense of
providing isolation guarantees to individual users. In addition, security is also another interesting issue to tackle.
Hierarchical storage: Though memory capacity grows exponentially each year, it is still comparatively expensive to
its alternatives. One early adopter of Tachyon suggested that
besides utilizing the memory layer, Tachyon should also
leverage NVRAM and SSDs. In the future, we will investigate how to support hierarchical storage in Tachyon.
Checkpoint Algorithm Optimizations: In this work, we
proposed the Edge algorithm to provide a bounded recomputation time. It also checkpoints hot files first and avoids
checkpointing temporary files. However, there could be
other techniques to make trade-offs among different metrics, e.g. checkpoint cost, single file recovery cost, and all
files recovery cost. We leave those as future work and encourage the community to explore different algorithms in
this area.
10.
Conclusion
As ever more datacenter workloads start to be in memory, write throughput becomes a major bottleneck for applications. Therefore, we believe that lineage-based recovery might be the only way to speed up cluster storage systems to achieve memory throughput. We proposed Tachyon,
a memory-centric storage system that incorporates lineage
to speed up the significant part of the workload consisting
of deterministic batch jobs. We identify and address some
of the key challenges in making Tachyon practical. Our
evaluations show that Tachyon provides promising speedups
over existing storage alternatives. Approaches similar to ours
are also being adopted by the HDFS community to leverage memory efficiently on clusters. We have open sourced
Tachyon at tachyon-project.org for people to use in real environments or as a system research platform. It has received
contributions from more than 40 individuals and over 15
companies.
11.
Acknowledgements
We thank Adam Oliner, Aurojit Panda, Calvin Jia, Daniel
Haas, Dilip Joseph, Evan Sparks, Ganesh Ananthanarayanan,
Gene Pang, Jiannan Wang, Joseph Gonzalez, Kaifei Chen,
Kay Ousterhout, Liwen Sun, Michael Franklin, Mosharaf
Chowdhury, Peter Bailis, Reynold Xin, Sameer Agarwal,
Shivaram Venkataraman, Tathagata Das, and our colleagues
in AMPLab UC Berkeley providing us many good suggestions. We also thank our shepherd, Pradeep Padala, and our
reviewers for their feedback. This research is supported in
part by NSF CISE Expeditions Award CCF-1139158, LBNL
Award 7076018, and DARPA XData Award FA8750-12-20331, and gifts from Amazon Web Services, Google, SAP,
The Thomas and Stacey Siebel Foundation, Adobe, Apple,
Inc., Bosch, C3Energy, Cisco, Cloudera, EMC, Ericsson,
Facebook, GameOnTalis, Guavus, HP, Huawei, Intel, Microsoft, NetApp, Pivotal, Splunk, Virdata, VMware, and Yahoo!.
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